CRT Projection Displays

CRT Projection Display

CRT’s are the most difficult type of display to calibrate and are often the most inaccurate out of the box. However, once a CRT based display has been given proper care their picture will surpass that rendered by any of the more modern display technologies. Quite simply, a display that uses CRT’s is still the most accurate and will have the most striking picture after calibration.

To calibrate a CRT usually requires between six to twelve hours. I work hard on these sets because I feel that the investment in time and labor really pays off. Once you see your CRT after it has been calibrated you will probably wonder how you could have ever looked at the picture the way it was before. The improvement is most dramatic.

Inspection

First and foremost, I inspect the TV for any defects that may impact the quality of the calibration. If I find any problems I will point them out. Assuming there is nothing wrong with the display I will then quickly get started.

Optics Cleaning

New or old, any projection television needs its optics cleaned.

One may assume that an older CRT’s optics will need cleaning. One may also think that their new TV came from the factory with clean optics. Unfortunately,, this is never the case. Manufacturers don’t seem concerned about the cleanliness of the optics even when you take your new set right out of the box. It is very standard for even brand new displays to have a large accumulation of dust on the lenses and mirrors and also for a dead spider or cricket to have crawled between the lenses and CRT’s. I have seen many cases where rear projectors have large spider webs in the cabinet passing right across the screen.

When I clean the optics I completely disassemble the lens stacks and remove the lenses from the set. I remove the dead bugs, debris, and clean the dust from the lenses. I also clean the mirror and inner screen.

Cut-Off Voltage
This adjustment is sometimes referred to as setting the G2’s or adjusting the VR’s. To set the cut-off’s requires the use of a digital multi meter or oscilloscope. This setting involves adjusting variable resistors that are often located behind the front panel. Proper setting of the G2’s will establish the correct CRT’s baseline voltage. Most importantly, properly adjusted cut-off voltage will extend CRT life. As a positive byproduct, setting the G2’s to spec may also improve gray tracking, stability of gray tracking, black details, white level, gamma, and color accuracy.

Mechanical, Electrical and Magnetic Focus
CRT displays usually have a few different types of focus adjustments that should be made. The most common three on CRT displays include mechanical, electrostatic, and magnetic focusing.

Mechanical is the actual focusing of the lenses. Adjustments to mechanical focus are made in the same way as you would adjust binoculars or a telescope; the lenses are rotated until proper focus is realized. Electrostatic focus involves adjustment of a variable resistor to really fine tune focus. Magnetic focus actually covers a range of focus options that may or may not be available or adjustable on any given display (Mitsubishi calls these adjustments “Quad Field Focus” while other manufacturers may call them magnetic centering ring focus). Generally what is accomplished with magnetic focus is to correct stigmatism errors and generally “true up” the image clarity. Changes to magnetic focus can have a most profound impact on the image.

All three types of focus will be typically out of alignment right from the factory. Obviously making focus adjustments will improve the sharpness and detail of the picture.

Geometry, Overscan, and Convergence
No CRT projector has very good geometry right out of the box. This is because geometry is really hard to adjust and the assembly line tech’s simply don’t have enough time to get these adjustments all that great.

On a CRT projector I usually spend between four to seven hours correcting and calibrating geometry. I make all geometry adjustments in the service menu so they will become the new factory defaults.

Typically, CRT projection displays will come from the factory with between 6% to 9% overscan. This factory overscan will also not be equal on all sides or the screen. I bring in this overscan to between 3% or 4% on RPTV’s and to between 1% or 2% on FPTV’s. Not only does this allow you to see more of the image, but it also greatly improves detail and sharpness. This is because by putting more of the image on the screen a “denser pixel pack” is created. What this means is that the “pixels” are closer together on the screen resulting in more detail.

Geometry also involves straightening out and linearizing the image. After the proper overscan is realized I will then spend even more time just straightening out the image to eliminate any swerves or bumps in the picture.

Finally, service level convergence will be performed. Service convergence is more accurate than the user convergence available in the user menu. Also, as you may have noticed, on many models if the TV is reset or if power is lost the user convergence may revert back to factory default. This results in the need for the user to redo convergence every time there is a power failure. The service level convergence that I perform becomes the new factory default and is permanent. Thus, your TV’s convergence will not be affected by power failures. It should not be forgotten that convergence not only aligns all three CRT’s but also is another step in greatly improving sharpness and detail.

To professionally perform geometry and convergence requires the use of an external calibration generator. I use my AccuPel HDG-3000 generator to produce patterns for these purposes. The internal convergence grid in the TV is not reliable and will usually not produce an optimum image. This is because the internal grid is generally not properly centered or linearized from the factory. Also, the red, blue, and green internal grids usually don’t properly align on top of one another. This makes it impossible to properly converge the image by using only the internal grid. Furthermore, a DVD player is not an ideal source to be used for geometry adjustments. This is because the “timing” in a DVD player will be non-standard. This causes improper image placement on the TV screen. When using a DVD player as a reference, the geometry will be correct for that particular DVD player, but future DVD players or any other sources will not be correctly aligned.

An external reference level pattern is required to ensure proper centering and linearization of the TV’s image. If a pattern generator is used, you can be certain that the timing will be correct and that the TV will then meet industry standards. This will allow most sources and DVD players to produce nearly perfect image placement.

Reprogram Display Software (Mitsubishi only)

In case you are unaware, an EPROM is a microchip that stores information much like a floppy disc does on a computer. A Mitsubishi TV has at least one EPROM containing information that controls the TV’s functions. I literally reprogram this software stored on the TV’s control EPROM. I use my own edited version of the Mitsubishi programming that I have developed and tested (other good calibrators also do this but be sure to ask as it is not at all common). This reprogrammed software includes a fix for red push and allows me to actually linearize and balance the color decoder even on the xxx13 and xxx15 series TV’s. It is impossible to properly balance and align a xxx15, xxx13, xx311, xx411, and many other earlier models, without the use of reprogrammed software.

I also make many other changes to the factory Mitsubishi software including but not limited to disabling dynamic picture and dynamic color. This drastically improves details in blacks and makes color more natural. I also reprogram the edge enhancement functions on these TV’s. The way Mitsubishi chooses to program these TV’s incorporates the use of artificial edge enhancements (EE). Unfortunately, these EE’s do not improve the detail of the image, but are actually used to make the TV seem sharper then a competing model on a showroom floor. In fact, these EE’s distort the image and drown out a huge amount of detail. Without correction, you may notice and be annoyed by these EE’s. Usually the longer you own and watch your TV the more you start to notice EE’s.

Color Decoder Alignment to SMTPE Standards
This is one of the most important steps in a calibration. Not only will I address the well documented red-push exhibited by most displays, but I also fix the lesser known problems of misalignment to blue, green, magenta, cyan, and yellow. I will fix the color decoder and make it conform closely to industry standards. Most calibrators make these adjustments by eye with the help of test patterns. I however use my Tektronix oscilloscope to ensure that the color decoder is as close as possible. These adjustments have a dramatic impact on the final image quality.

Brightness and Contrast
Something so easy as brightness and contrast can be overlooked. Perhaps this is because it’s actually not so easy to make these adjustments correctly. It really is difficult to tell just how dark or bright a picture is by eye, even when looking at test patterns.

I use my Accupel calibration generator along with my Sencore light meter to set and maintain both proper brightness and contrast. This is paramount for what will come next in establishing a nice gamma curve and allowing gray to track linearly. Also, setting proper contrast will extend CRT life and lessen the likelihood of burn-in.

Sharpness
Again, most calibrators make this adjustment by eye. I however use my oscilloscope along with my Accupel to set the exact sharpness level that is technically correct. This is not easy to do by eye as it can be subjective. However, with the use of a multiburst test pattern and an oscilloscope it’s quite straight forward. Setting the sharpness to the technically correct level will maximize image detail without inducing distortions to the image such as ringing.

Gamma

Gamma is one of the least understood and most important adjustments to be made in a calibration. Honestly, I think adjusting gamma goes right over the heads of a lot of calibrators. I know of many who simply don’t worry about it. Setting gamma correctly will have a most profound impact on the image. Without proper gamma adjustment black details will be lost and the image will either appear too washed out or way to dark. No matter how accurately brightness and contrast are set, if gamma is in error the picture will still have fundamental problems.

Gamma calibration is not so straight forward so I customize gamma based on your viewing habits. I use my Sencore to take light readings and make adjustments throughout the entire light emission spectrum. Industry standards dictate that gamma should be maintained between 2.2 and 2.5. If you tend to watch in a darker room then you will prefer gamma closer to 2.5. Inversely, if you have a room with bright lighting or large windows then you will likely prefer gamma closer to 2.2.

On some models (most Mitsubishi’s) I can even set different gamma curves for each scan rate. This way if you tend to watch 480i SD during the day in a bright room but often watch 480p DVD at night in a dark room, I can get an optimum curve for both lighting conditions. In this example I would target gamma for SD to 2.2 but target gamma to 2.5 for DVD.Calibrate Gray Scale to D65
Gamma, brightness, contrast, and other settings will change and interfere with gray tracking. Therefore I try and leave gray scale calibration for the final adjustment. Only after all of the above procedures are completed will I then calibrate gray scale. For television, I set gray scale to the industry standard of 6500 degrees Kelvin. This is a difficult task and usually takes about two hours. If your display requires a standard other then D65 I am capable of providing this as well.

When most people hear about setting gray scale they don’t normally really understand what it does. I think people assume that it’s not very important or may only affect objects on the screen that are gray or black or white. However this is definitely not the whole story. The most profound impact proper gray scale calibration will have on the image is to dramatically improve the richness and realism of colors. This is because colors in a color display are only imposed on top of black and white images. Thus, if you have color in your black and white picture this color will be skewed in the final image.

Typically manufacturers will set gray scale to accentuate a color in order to make their display stand out on the show room flour. This is apparent because black and white images look colored (often blue). However, when color video is watched this extra color in the gray scale will be apparent in the image by skewing the color and exaggerating it. Once gray is calibrated all images will become more natural and have fuller color but at the same time the image will not look over saturated. Gray scale calibration is one of the most import procedures in any video display calibration.

Time and Pricing

This procedure is considered a full calibration and costs US$625 on most rear projection CRT displays. Front projection CRT calibrations usually follow a very similar procedure but will require more time. As such their cost will generally be a little higher (email for details). For Rear projection CRT’s I require between six to twelve hours to complete a full calibration. As indicated, any new or old CRT will show tremendous benefit from a full calibration.

I promise you will be satisfied with the results before I leave. I provide top notch service and come equipped with the best equipment available to calibrators today. Please fill out my appointment form to schedule a calibration soon. Also, if you have any further questions feel free to send me an email craigr@cir-engineering.com.Optional Services (Mitsubishi only)